The present invention relates to a technology for registration between medical images.
As imaging apparatuses, there have heretofore been known, for example, a magnetic resonance apparatus (MR), a radiation tomography imaging apparatus (CT: Computed Tomography), an ultrasound apparatus (US: Ultra Sound), etc. These imaging apparatuses respectively have merits/demerits existing for every imaging modality and may be deficient in diagnosis accuracy only in an image by a specific imaging modality. Therefore, in recent years, attempts have frequently been made to improve diagnosis accuracy by performing a diagnosis using not only the image by the specific imaging modality but also images by a plurality of different imaging modalities.
In the diagnosis using the images by the different imaging modalities, image coordinate systems differ for every imaging modality. Therefore, a technology for correcting positional displacements due to the difference between these coordinate systems and variations/deformation of each organ (i.e., a technology for registration between images) is important.
Meanwhile, as a method of performing registration between a plurality of images different from each other, a method using mutual information is the most common (refer to, for example, IEEE Trans. on Med. Imaging, 16:187-198, 1997). This method is of an intensity based method based on the brightness value of an image in a broad sense. That is, it is a prerequisite that object images have to do with brightness values thereamong to perform registration using the mutual information.
In a US image, however, an acoustic shadow occurs and hence a brightness value at the back of a high reflector is lowered more than the original value. The brightness value of each blood vessel also changes depending on the running direction of the blood vessel. Therefore, for example, in the registration between an MR image and a US image or the registration between a CT image and a US image, the situation in which brightness values have poor relevance often occurs, and there is a case in which the accuracy of registration becomes extremely deteriorated.
With the foregoing in view, there has been a demand for a technology capable of performing registration between medical images with higher accuracy.